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ABSTRACT Numbers of European glass eels (_Anguilla anguilla)_ monitored along the Atlantic and Mediterranean coasts of Europe currently serve as the main stock indicator in assessment of

this critically endangered species. Spawning, however, takes place exclusively in the Sargasso Sea, several thousand kilometers away. The beginning of its complex lifecycle is characterized

by a distant and lengthy larval drift, before the young-of-the-year reach the monitoring stations at the European coasts. The oceanic mechanisms regulating dispersal and distribution of

European eel leptocephalus larvae, before they metamorphose into glass eels and colonize future growth habitats, are still poorly understood and data are scarce. Here, we present

oceanographic and leptocephalus catch data from a 24-h station on board of the German Research Vessel Meteor, covering one event cycle of the tide-derived change of hydrographic conditions

in the central part of the Strait of Gibraltar. Results of this study provide detailed insights on how the exchange of water masses between the Atlantic and the Mediterranean Sea may favor

or prevent transport and migration of eel larvae through the Strait, which potentially plays a decisive role in timing and magnitude of larval recruitment events into the entire

Mediterranean region. SIMILAR CONTENT BEING VIEWED BY OTHERS NORTH EQUATORIAL CURRENT AND KUROSHIO VELOCITY VARIATIONS AFFECT BODY LENGTH AND DISTRIBUTION OF THE JAPANESE EEL _ANGUILLA

JAPONICA_ IN TAIWAN AND JAPAN Article Open access 21 February 2022 FIRST DIRECT EVIDENCE OF ADULT EUROPEAN EELS MIGRATING TO THEIR BREEDING PLACE IN THE SARGASSO SEA Article Open access 13

October 2022 NEW CLUES ON THE ATLANTIC EELS SPAWNING BEHAVIOR AND AREA: THE MID-ATLANTIC RIDGE HYPOTHESIS Article Open access 06 October 2020 INTRODUCTION The stock of the panmictic European

eel (_Anguilla anguilla_) exhibited substantial decline in recent decades and is currently rated as critically endangered by the IUCN Red List of Threatened Species1. Recruitment

time-series for glass eels and yellow eels, monitored along the Atlantic, Baltic and Mediterranean coastlines, currently serve as the basis for the evaluation of the status and development

of the stock2. While these data provide crucial insights into recruitment dynamics, they do not capture all potential threats or changes affecting the whole stock, emphasizing the need for a

comprehensive understanding of the complete life cycle including the leptocephalus stage3,4,5. Especially early life history stages such as eggs and larvae are vulnerable to a number of

additional oceanic and environmental factors6,7,8,9,10,11,12. Investigating and surveying changes in oceanic abundance and distribution of larvae can be valuable to define the role of

oceanic factors in the species decline and provide information on the temporal and regional fluctuations in coastal recruitment7,13. Further, knowledge about the transport and dispersal

mechanisms of leptocephalus larvae in the open North-West Atlantic, but also in the coastal areas after their journey from the Sargasso Sea, are key to understand and predict the magnitude

of yearly recruitment in different areas14. Ocean currents play a decisive role in timing as well as trajectory of the drifting leptocephalus larvae, as distinct current patterns have been

identified that either favor a transport to the North Sea or towards the Mediterranean Sea4,11,15,16,17,18. Besides oceanographic features, larval behavior can also be a key element in this

regard since anguillid leptocephali are not seen as fully passive drifters and observations as well as behavioral studies have shown their ability to actively swim horizontally and to rise

and dive in the water column19,20,21. Even though availability of empiric data on the vertical distribution of leptocephalus larvae in the water column is not extensive, current

understanding is that larger, later stage anguillid leptocephali conduct diurnal vertical migrations (DVM) and are usually found in deeper water layers during daytime but in shallower,

surface-near layers during the night13,20,22,23. Previous studies described the preferred depth during nighttime to range between 50 and 70 m, while the extent of vertical distribution

during daylight seems to increase with body length24. Tesch23 reported large leptocephali close to the European shelf to reside mainly between 300 and 600 m during daytime. Previous studies

have shown that this distinct behavior also mirrors in the catchability of the larvae in certain depth ranges, which is the reason why the sampling strategies and targeted depth strata for

specimens of this life history stage usually differ during night and day22,25. The vertical position of the larvae during different times of day and night thus potentially affects their

oceanic distribution, as direction and velocity of currents often vary depending on the depth stratum. Not only due to the complexity of larval oceanic dispersal and the wide natural

distribution, which increase the efforts to develop robust monitoring strategies, there remains a paucity of data on recruitment and standing stock in certain parts of the eel’s range. Glass

eel monitoring at selected sites in the Mediterranean and close to the Strait (e.g. at the Guadalquivir River, located on the Spanish Atlantic side) are considered crucial data points to

describe recruitment trends in the Mediterranean Sea26. Still, the Mediterranean Sea is particularly considered inadequately covered by recruitment time-series, even though the area is

thought to be an important habitat for the species. Until today, it is neither entirely clear in what magnitude its production contributes to the spawning stock biomass, nor precisely how

high the natural recruitment is in this area2,13,26,27,28. The Mediterranean Sea and the Atlantic Ocean are connected by the Strait of Gibraltar, a narrow (∼13 km at its narrowest point) and

rather shallow (∼280 m at the shallowest point, the Camarinal Sill) bottleneck, which young eels inevitably need to pass in order to colonize any coastal area or river system of the

Mediterranean or Black Sea. This, of course, is also true for adult silver eels emigrating out of the Mediterranean, in order to successfully conduct their spawning migration24,27,29.

Similar to other gateways between different ocean basins, water exchange, current and tidal systems in the Strait of Gibraltar are complex and regulated by a variety of factors24.

Semi-enclosed basins, such as the Mediterranean, typically display differences in several physical characteristics from the open Atlantic Ocean29. The water balance of the Baltic Sea for

instance, is controlled by outflow of a freshwater and precipitation-fed brackish water column within the upper layer30, while inflow is characterized by high saline water from the Kattegat

in deeper strata31. In contrast, the Mediterranean Sea is considered a concentration basin, which is characterized by water loss due to evaporation exceeding water gain from precipitation

and river runoff. Adding to this, there is a negative net heat budget of the basin, causing an anti-estuarine vertical thermohaline circulation, as deeper, more saline water exits the

Mediterranean, while surface waters from the Atlantic enter the Mediterranean32,33. Additionally, these density-driven currents are superimposed by tidal currents that can exceed two meters

per second and thus strengthen or slow down the eastwards-directed (i.e., towards the east sector, including east, east-northeast, and east-southeast directions) surface inflow, depending on

the phase of the tide. The tidal energetics of the Strait of Gibraltar are complicated and have been intensely studied33,34,35,36,37. However, even though notable scientific effort in

collecting data for this region was made, for a long time, the tidal regime of the Strait was considered still not entirely understood by some authors38. The current flow is assumed to

interact with the Camarinal Sill (shallow part of the Strait) causing surface wave patterns, that are detectable by satellites35,39. The fluctuating conditions and temporarily strong

currents in the Strait in connection with the limited mobility of the few centimeters long leptocephali suggest that recruitment and transport from the Atlantic into the Mediterranean must

be linked to specific conditions, as was already supposed by Tesch et al. in 198640. The significance of the area for the distribution of eel larvae was also highlighted in a dedicated

paragraph in McCleave et al.24, who hypothesized that the tidal conditions in the Strait make a passive transport of relatively shallow residing leptocephali during spring tide not only

possible, but likely. In more detail, and considering the behavioral characteristics of eel larvae (DVM) as well as the oceanographic conditions in the Strait, it was hypothesized that

optimal conditions for the larval transport into the Mediterranean Sea would only be given, when strong eastward surface currents occur at nighttime after sunset, when the eel larvae ascend

to the upper part of the water column. The here presented study builds on this work by providing oceanographic and larval catch data during a 24-h station to complement the hypothesis with

concrete data. The aim of this study was therefore to gain better insight into the transport of anguillid leptocephali at a highly dynamic bottleneck during their transoceanic larval

migration. Results are based on periodical plankton sampling with an Isaacs-Kidd Midwater Trawl (IKMT) in the central part of the Strait of Gibraltar. During a full tidal cycle, we monitored

velocity in the water column using acoustic doppler current profiler (ADCP), additional hydrographic parameters using a conductivity, temperature and depth rosette (CTD) as well as surface

current velocity and direction derived from high-frequency radar (HFR). RESULTS Seven deployments of the IKMT during 24 h in the central part of the Strait of Gibraltar (Fig. 1) revealed

clear differences in leptocephalus catch quantities. While zero larvae were caught during the two daytime and one evening hauls in depths from 0 down to 200 m, a total of 31 leptocephali

including 17 _Anguilla anguilla_ (total length (TL): 54–65 mm), 12 _Conger conger_ (TL: 50–71 mm), 1 _Eurypharynx pelecanoides_ (TL: 16 mm) and 1 _Gnathophis_ sp. (TL: NA) specimen were

caught in the upper 100 m during the four nighttime hauls. A total of 13 _A. anguilla_ and 4 other leptocephali were caught during one (G5) of the deployments on November 21st beginning at

01:00 UTC. This haul produced the highest catch per unit effort (CPUE) of 9.51 caught larvae per hour, as well as the highest calculated larval density (3.31*10−4 individuals / m3) in this

study and the entire survey41 . WEATHER AND OCEANIC CONDITIONS Wind direction and speed (Table 1) during the observed time frame were moderate and tidal current velocities can be considered

common for the Strait at this time of the year. The tides on November 20th and 21st, 2022 occurred during a waning moon, meaning that sampling did not take place during extreme tidal ranges

like those seen during spring tides (Table 2). Higher windspeeds and/or spring tide conditions would have had an increasing effect on the currents in the Strait. WATER STRATIFICATION IN

OBSERVED TIMEFRAME CTD data (Fig. 2) confirmed a stratified water column in the converging area in the central part of the Strait of Gibraltar, which corresponded well with the known

separation of water masses from the Atlantic and the Mediterranean Sea. While the surface layer was characterized by a 70–100 m deep lens of warmer, less saline and less dense water, the

deeper layers of the water column were characterized by more saline, colder and thus denser water masses. In addition to these characteristics, higher oxygen levels in the surface layer

indicate a stronger mixing in this depth stratum. The CTD profiles at 00:04, 06:00 and 13:16 UTC (Fig. 2) reflect the tidal influences on the water stratification (Fig. 3). The profile at

00:04 (purple) was recorded shortly after high tide and showed lower temperatures and higher salinity between 37 and 250 m depth due to the westward transport of Mediterranean water. In

contrast, the profile at 06:00 (orange) was recorded after low tide, with the influence of eastward flowing Atlantic water leading to higher temperatures and lower salinity in the same depth

range. The profile at 13:16 (black), taken after the second high tide, was consistent with the observed westward transport below 300 m. These fluctuations, which are supported by ADCP

measurements (Fig. 3), illustrate the influence of tidal cycles on the distribution of water masses. WATER CURRENT VELOCITY AND DIRECTION MEASURED BY ACOUSTIC DOPPLER CURRENT PROFILER (ADCP)

For technical reasons, the shipboard ADCP velocity measurements started with haul G2 in the afternoon of November 20th and (in correspondence with CTD data) also showed a stratified current

structure in the water column. Strong variation was present in the eastward velocity component (Fig. 3a,c) that flowed from the open Atlantic towards the Mediterranean Basin and rather

negligible variation in the northward velocity components (Fig. 3b,d). For the eastward component, current velocities generally differed during the observed timeframe depending on depth,

with highest velocities in the upper 100 m, reaching a maximum of almost 200 cm s−1 due to strong inflow from the Atlantic into the Mediterranean (Fig. 3a). High velocities of more than 120 

cm s−1 were observed at the end of the first monitored outgoing tidal cycle (20/11/2022, approx. 16:20 UTC) until shortly after the first low tide (20/11/2022, approx. 16:50 UTC), as well as

between the first high tide until after the second low tide of the observed timeframe (21/11/2023, approx. 00:15–08:00 UTC). This is highlighted in Fig. 3c for the exemplary depths 37 m

(black) and 101 m (violet), starting around 00:00 UTC on November 21st until after low tide, approx. 08:00 UTC on November 21st, which was shortly after the last haul (G7). Deeper water

strata, exemplified here by the depth of 197 m (orange), show a negative eastward movement (i.e., a westward movement) during most of the tidal cycle, and neutral or only very slight

eastward movement occurring between high tide and low tide (Fig. 3a,c). ADCP vertical profiles of water velocity magnitude and direction during the timing of the most successful haul (G5)

depict the magnitude and direction of the inflow event in eastward direction (Figs. 3, 4c). Water profile plots of current speed and direction at three exemplary times (beginning, mid-haul,

end of haul) of hauls with caught anguillid larvae (G3 (n = 2), G4 (n = 2) and G5 (n = 13)) further depict the respective prevailing current regimes in different depths (Fig. 4a–c). While

current direction and speed during the first haul with caught larvae in the upper water layer (37–100 m) in the beginning and mid haul was directed eastward to southward at a moderate

velocity ranging from 25 to 50 cm s−1 (Fig. 4a), the water turned directions amidst the haul completely westwards through the entire water column from surface down to 600 m (Fig. 4a). The

second haul with caught larvae (Fig. 4b) displayed a westward direction in the very upper water layer and a south-western trajectory in the beginning and mid haul. Current speed was slow to

moderate, ranging from approx. 10–76 cm s−1, with peaks around 100 cm s−1 between 100 and 165 m of depth. The highest observed current velocities were seen in G5, the haul with the highest

count of collected anguillid leptocephali (Fig. 4c). During the three exemplary times of this haul, current velocity reached 175 cm s−1 in the upper 100 m of the water column in an eastward

direction. Current direction in greater depths during this haul turned from north-eastwards to eastwards at low to moderate speed ranging from 0 to 40 cm s−1 (Fig. 4c). ATLANTIC JET SEA

SURFACE CURRENT DERIVED FROM HIGH-FREQUENCY RADAR Data derived from high-frequency radar monitoring sea surface currents in the Strait of Gibraltar show a strong inflow event from November

20th, 2022 at 23:00 UTC until November 21st, 2022 at 08:00 UTC (Fig. 5). High eastward velocities (yellow & red) characterized the Atlantic jet current, which moved surface water masses

eastwards through the Strait of Gibraltar from the Atlantic into the Mediterranean (Fig. 5c,d). DISCUSSION This study is the first to provide catch data and environmental data as connected

evidence of how eel larvae are transported from the Atlantic to the Mediterranean Sea. We provide further support for the hypothesis by McCleave et al.24 that optimal conditions for larval

transport into the Mediterranean Sea arise when strong eastward surface currents occur at night, coinciding with the ascent of eel larvae into the upper, shallower layers of the water column

as part of their diel vertical migration behavior. Utilizing this ‘perfect wave’ of strong eastward currents appears to facilitate the most effective transport of larvae. However, it is

important to note that the necessary conditions are not uniform throughout the year. In winter, southern Spain experiences longer periods of darkness (nearly 14 h) compared to summer. This

extended darkness aligns with tidal cycles, providing more potential windows for larval eel transport than in summer. The exact timing of larval transport within the water column, and

whether it occurs right after sunset or a later at night, may affect their interaction with tidal conditions. If larvae migrate upwards shortly after sunset and return to deeper waters at

sunrise, winter conditions with longer nights could offer more favorable conditions for transport compared to summer, especially during spring tides or strong westerly winds. Miller et al.4

published a comprehensive review on the distribution, timing and size of _A. anguilla_ leptocephali, documenting that late stage leptocephali have been collected almost throughout the entire

year around the Strait of Gibraltar. It is unclear if the abundance of leptocephalus larvae in this area greatly changes throughout the year, or if there is a potential peak of arrival at a

specific timeframe. But while it seems reasonable to assume that there might be a temporal connection to a time before the peak arrival of glass eels in estuaries around the Strait and in

the Mediterranean, it remains unclear if and to what extent larval abundances in this area change throughout the year. Relating the ADCP and HFR data with the IKMT catches of this study,

larvae were only collected at nighttime and highest leptocephalus abundance (G5) was recorded during the formation of an eastward directed wave of Atlantic water towards the Mediterranean

(Figs. 2, 4c). The other two hauls (G3 and G4) containing _Anguilla_ larvae were conducted during non or only slowly westward flowing incoming tide, as can be seen in Fig. 4. These larvae

were possibly in the surface strata of a water mass that has not been entirely pushed through the Strait but instead was (at least in parts) pulled back to the western Atlantic part of the

Strait when the tides changed. The outflow of the heavier, more saline water out of the Mediterranean Sea through the Strait in westward direction possibly constitutes a natural barrier for

leptocephalus larvae in front of the Strait. Miller20 underlined how poorly documented the swimming capabilities of leptocephalus larvae were. Even though it cannot be ruled out that

anguillid eel larvae are capable of single-direction swimming to increase recruitment success19, their capability of actively swimming long distances or against strong currents has been

subject to scientific discussion and speculation in the past4,15,24,40. Only few publications provide empiric data on the swimming capacity of anguillid larvae: Artificially hatched 30–60 mm

long larvae of the Japanese eel (_Anguilla japonica_) reached swimming speeds of 3.6 + /- 2.7 cm s−1 horizontally and 2.8 + /- 1.1 cm s−1 vertically in captivity, which corresponds roughly

to one body length s−121. Interestingly, this constitutes a swimming speed that has shown the best hypothetical recruitment success in models, compared to inactive drifting42. Swimming

speeds of late stage or metamorphosing _Conger oceanicus_ leptocephali and _A. rostrata_ glass eels, measured in a laboratory swimming channel, were faster: ranging from 12.0 to 26.8 cm s−1

and 4.1 to 25.0 cm s−1, respectively43. During the 24 h observation period, current velocities in deeper, westward flowing water layers ranged from approx. 50–100 cm s−1, except during the

tidal jet surface inflow associated with Atlantic high tides. These current velocities, compared to the known swimming speeds of larvae, suggest that active swimming of anguillid

leptocephali against the outflowing currents of the Mediterranean water during daytime over a longer time span and distance must be deemed unrealistic. Tesch et al.40 were apparently the

first to address this question after a larval survey in the area and described a stowing or bottleneck effect in the funnel between Europe and Africa, just before the Strait of Gibraltar.

The authors also concluded that a swimming speed of at least 20 cm s−1 was required to effectively compensate westwards outflowing currents from the Mediterranean. Our data support this, as

the observed tidal dynamics, which are characterized by fast eastward-flowing surface water strata and stationary or slowly westwards-moving deeper layers could explain accumulation effects

of leptocephali west of the Strait. These accumulations likely persist until the next eastward tidal pulse transports the larvae during their shallow nighttime phase of the DVM. Current

direction and velocity in the Strait of Gibraltar are greatly influenced by the Atlantic tidal system, resulting in a constant move of oceanic water from the Atlantic into the Mediterranean

and back. Our recordings during the 24 h station in the Strait are consistent with a model that describes the exchange of water masses between the oceanic bodies as tidal oscillation waves

with movement patterns in different directions, yet mainly following an eastward trajectory44. Izquierdo et al.45 considered tides to be by far the most energetic process in the Strait, that

present a semidiurnal character, though diurnal current velocities were not negligible. These tides are essential for the water movement in the Strait all year round, so although there was

not much wind on this occasion, the observed conditions during the 24 h station can be regarded as representative of average conditions in the Strait of Gibraltar. While most of the time

heavier, more saline water pushes out in deeper layers westwards of the Mediterranean Sea into the Atlantic, high tides and rising water levels in the Atlantic facilitate intensification of

eastward surface currents, pushing into the Strait (e.g. as can be seen in the eastward directed wave in Figs. 2c, 4a–d) with velocities of close to 200 cm s−1 (e.g. 7.2 km h−1 or > 20 km

in three hours tidal phase) and more in the upper 50–100 m of the water column. As during these conditions, also the deeper water layers move eastwards (Fig. 4c), this (albeit considerably

slower) could enable recruitment into the Mediterranean also in greater depths during daytime, when the larvae still linger in deeper waters. Given that the European eel is a panmictic

species, a selective and directed recruitment into the Mediterranean Sea seems unlikely. Yet, the mechanisms, timing and triggers that initiate leptocephalus larvae to metamorphose into

glass eels and colonize continental habitats remain poorly understood. While for glass eels, it was shown that a geomagnetic sense could guide them to use tidal cycles and orientate towards

the coastal shores to colonize coastal and freshwater habitats46, it is unknown whether eel leptocephali have similar capabilities to guide their transoceanic migration47,48. Regardless of

this, larval as well as metamorphosing eels exhibit negative phototaxis, as demonstrated for captive reared Japanese eel larvae, for which overhead light caused clear downward movement in

experimental works21. In the same artificially spawned larvae, Yamada et al.21 observed that swimming speeds during horizontal movement and rising behavior were significantly higher than

during diving behavior and suggested that this may help to regulate distribution during inshore migration of young arriving recruits. Tidal waves pushing surface water through the Strait

into the Mediterranean Sea during nighttime could therefore be regarded as a transport pump that will directly affect dispersal of eel larvae and other organisms conducting nocturnal

vertical migrations into the surface strata of the water column. Macias et al.34, who studied the influence of internal waves on the composition and abundance of zooplankton in the Strait,

already highlighted the need to understand and discuss the physical components and processes when addressing biological patterns in regions with intense hydrodynamic regimes. Despite some

limitations due to the confined timeframe of this study, the obtained data and observations are consistent with expectations and the hypotheses presented herewith. Further research covering

longer timeframes ideally throughout other times of the year together with depth-stratified sampling would help to test the here presented hypotheses. Future studies should include

consideration of peak arrival of recruits in the area, environmental factors such as weather conditions, different tidal phases and changing environmental conditions caused by climate

change. CONCLUSIONS This study describes how the interaction between diurnal vertical migration behavior of leptocephalus larvae and the tidal conditions in the Strait of Gibraltar likely

enables the recruitment of European eel larvae into the Mediterranean Sea. The presented data suggest that a significant eastward surface current in combination with the ascent of

leptocephali into the upper water layers at nighttime effectively transports larvae from the Atlantic Ocean to the Mediterranean Sea via the Strait of Gibraltar. Consequently, the here

presented data contribute to our knowledge about the hydrographic factors determining the magnitude and timing of early life stage recruitment events into the Mediterranean and could help

develop convenient targeted and effective recruitment monitoring strategies for this area, complementary to glass eel recruitment time-series. METHODS The collection of oceanic data, current

parameters, as well as the sampling of leptocephalus larvae, was conducted in November 2022 during one leg of the 185th cruise (M185) aboard the German Research Vessel METEOR. LEPTOCEPHALUS

CATCH DATA All leptocephali in this study were collected using a 1550 µm mesh-size Isaacs-Kidd Midwater Trawl (IKMT) with a ~ 7 m2 mouth opening and a length of 12 m (HYDRO-BIOS Apparatebau

GmbH, Altenholz, Germany). Seven hauls were conducted during a 24-h station between the 20th and 21st of November 2022 in the middle of the Strait of Gibraltar, located roughly at 35°56 N

and 5°37 W (Fig. 1; Table 1). The ships speed over ground was adapted to current speed in order to facilitate the gear’s ideal and comparable trawling speed of 2.5 knots in water. As a

result, starting and ending points but also the timing of vertical positions in each haul varied (See Fig. 1 & S1 for details). Filtered water volumes were assessed with flowmeters

(2030R, General Oceanics, Miami, USA). As stratified sampling was not possible during this cruise, the IKMT was towed in three double oblique hauls between surface and 200 m of depth during

daytime and early evenings and in four quadruple oblique hauls between surface and 100 m of depth during nighttime and early morning, to target only surface-near leptocephali after or during

their diel vertical migration (See Table 1 & S1 for details). This was done to be able to appoint the caught larvae to the upper water strata. Fishing at deeper strata would have

potentially captured deeper larvae and thus may have led to misinterpretation of larval vertical distribution. Collected larvae were immediately sorted from the catch, morphologically

identified, measured, tissue-sampled for genetic species determination and then stored at −20 ℃ for further investigations. All leptocephalus larvae sampled at this station were genetically

confirmed by DNA barcoding. GENETIC SPECIES CONFIRMATION Tissue samples from each leptocephalus larva were separately preserved in ethanol (96% abs.) on board for subsequent genetic

analysis. After the survey, DNA was extracted using Chelex10049 and stored at 4 °C for several weeks or at −80 °C for long-term storage. The verification of morphologically identified

Anguillid species was carried out by a Real-Time PCR-based assay, detecting _A. anguilla_ and _A. rostrata_in comparison to reference samples. All remaining leptocephalus larvae were

morphologically identified, with their species identity confirmed through Nucleotide BLAST50 analysis of the mitochondrial DNA barcoding marker gene Cytochrome c oxidase I (COI), which was

amplified by PCR and sequenced using Sanger sequencing (Service lab: StarSEQ GmbH, Mainz, Germany). OCEANOGRAPHIC DATA As a standard procedure, prior to every deployment of the IKMT during

the 24-h station, vertical observations of temperature (℃), salinity (PSU), water density (kg m−3) and oxygen (mL L−1) were collected by using a SBE 911 conductivity temperature-depth

measure (CTD) (Sea-Bird Electronic, Bellevue, WA, USA). All CTD casts were conducted from sea surface to 600 m, respectively. ADCP DATA Ocean current velocities (from approx. 37 m to the sea

floor, depending on the region and sea state) were collected along the cruise track by a vessel-mounted Teledyne RD Instruments 38 kHz Ocean Surveyor ADCP (Acoustic Doppler Current

Profiler). The shipboard ADCP provides estimates of the horizontal velocity components as a function of depth, based on the Doppler effect of sound wave reflections against small particles

carried by ocean currents. The transducer was located at 5 m below the water line, mounted in the hull of the ship. The instrument was operated in narrowband mode with 16 m bins and a

blanking distance of 16 m, while 100 bins were recorded using a pulse of 2.89 s. The measurements obtained were averaged over 2 min. For more details, see Hanel et al.51. The maximum depth

reached in the Strait of Gibraltar was approx. 626 m. HIGH-FREQUENCY RADAR High-frequency radar (HFR) data was derived from in situ instruments operated by Puertos del Estado in the Strait

of Gibraltar. The HFR system deployed in Strait of Gibraltar consists of four sites equipped with a CODAR-Seasonde model. The sites, owned and operated by Puertos del Estado, started data

collection in May 2011. The system works at a central frequency of 26.8 MHz, providing quality-controlled observations representative of current velocities in the upper 0.5 m of the water

column52. The maximum horizontal range is set to 40 km, and the nominal range and angular resolutions are 1 km and 5°, respectively. This land-based network has proved to be useful in the

characterization of the Atlantic jet53. For further details about the fundamentals and applications in the Mediterranean Sea of this consolidated remote-sensing technology, see Lorente et

al.54. DATA AVAILABILITY The original datasets generated and analyzed during the current study will be made publicly available in the Figshare repository, accessible via

[https://doi.org/10.6084/m9.figshare.c.7583429.v1], upon publication. All relevant data are available from the corresponding author on reasonable request. REFERENCES * Pike, C., Crook, V.

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references ACKNOWLEDGEMENTS The scientific crew of M185 gratefully acknowledges the very friendly and most effective cooperation with Captain Detlef Korte and the entire METEOR crew. We also

thank Benedikt Merk, Silvia Blum, Peter Müller, Greta Voss, Kira Kremer and Sebastian Weis for their help on deck and in the laboratory. Many thanks also to the two reviewers in the

peer-review process, who have greatly helped to improve this publication with their pertinent and valuable comments and suggestions. Further, we appreciate the valuable support of the German

Research Fleet Coordination Centre (Leitstelle Deutsche Forschungsschiffe) at the University of Hamburg. FUNDING Open Access funding enabled and organized by Projekt DEAL. The R/V METEOR

expedition M185 was funded by the Deutsche Forschungsgemeinschaft—DFG. J.D.P, L.M. and M.F. are co-funded by the European Commission’s Data Collection Framework (DCF). AUTHOR INFORMATION

AUTHORS AND AFFILIATIONS * Thünen Institute of Fisheries Ecology, Federal Research Institute for Rural Areas, Forestry and Fisheries, 27572, Bremerhaven, Germany Marko Freese, Lasse Marohn, 

Jan-Dag Pohlmann, Klaus Wysujack, Tina Blancke & Reinhold Hanel * AZTI, Marine Research, Basque Research and Technology Alliance (BRTA), Herrera Kaia, Portualdea z/g, 20110, Pasaia,

Spain Luis Ferrer Authors * Marko Freese View author publications You can also search for this author inPubMed Google Scholar * Lasse Marohn View author publications You can also search for

this author inPubMed Google Scholar * Luis Ferrer View author publications You can also search for this author inPubMed Google Scholar * Jan-Dag Pohlmann View author publications You can

also search for this author inPubMed Google Scholar * Klaus Wysujack View author publications You can also search for this author inPubMed Google Scholar * Tina Blancke View author

publications You can also search for this author inPubMed Google Scholar * Reinhold Hanel View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS

M.F. and L.M. conceptualized the study. M.F., L.M., L.F., T.B., J.D.P., K.W., and R.H. developed and used the methodology. M.F., L.M., L.F., and T.B. performed the formal analysis. M.F.,

L.M., K.W., L.F., and R.H. provided resources. M.F., L.F., T.B., and L.M. curated the data. M.F., L.F., J.D.P., and T.B. prepared the visualizations. M.F. wrote the original draft of the

manuscript. All authors reviewed and edited the manuscript. CORRESPONDING AUTHOR Correspondence to Marko Freese. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing

interests. ETHICAL STATEMENT Animal ethical approval was not applicable for sampling in this study. ADDITIONAL INFORMATION PUBLISHER’S NOTE Springer Nature remains neutral with regard to

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of European eel larvae through the Strait of Gibraltar into the Mediterranean Sea. _Sci Rep_ 15, 1006 (2025). https://doi.org/10.1038/s41598-024-82929-z Download citation * Received: 13

September 2024 * Accepted: 10 December 2024 * Published: 06 January 2025 * DOI: https://doi.org/10.1038/s41598-024-82929-z SHARE THIS ARTICLE Anyone you share the following link with will be

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initiative KEYWORDS * _Anguilla anguilla_ * Recruitment * Leptocephalus * Tidal cycles * Internal waves